As a result of damage to the surface of the optical components of the powerful excimer lasers that operate at 248 nm, 193 nm, and 157 nm and are used for optical lithography, medical and industrial applications, the lasers must either operate at power levels lower than their maximum or shorter optical component lifetimes must be accepted if the lasers are operated at higher powers levels. For excimer laser optical components, metal-fluoride optical crystals of MgF2, BaF2 and particularly CaF2 are preferred materials due to their excellent optical properties and high band gap energies. Oxide materials are not a good choice for optical components because they are either too absorbing or they have low band gap energies, and are therefore vulnerable to color center formation when subjected to wavelengths below 200 nm. In addition, oxide materials such as high purity fused silicon dioxide have also been shown to be subject to material compaction at short wavelengths below 200 nm.
Various excimer laser components may require anti-reflection, mirror or partial mirror coatings. These coating materials are typically placed on the optical components using vacuum deposition techniques; and the materials used for such coating are also made of metal-fluoride materials. Examples of the materials used for such coating include AlF3, NaF, MgF2, LaF3, GdF3, and NdF3 among others. These optical coatings are generally considered to be the weakest links or component features in the laser systems due to the fragile and porous nature of depositions. As a result, the optics and coatings are typically used in operation in an atmosphere nitrogen purge gas to minimize problems from atmospheric contaminants as well as atmospheric absorbance in the beam path. However, even in well purged environments metal-fluoride optical coatings have been shown to degrade [See V. Liberman et al., “Ambient effects on the laser durability of 157-nm optical coatings,” SPIE Vol. 5040, (2003) pp. 487-498] and uncoated metal-fluoride surfaces will similarly degrade, for example, by reaction with atmospheric moisture and carbon dioxide.
Laser manufacturers have attempted to minimize coating damage by using techniques as leaving the optical component's surfaces uncoated and/or tilting the surfaces to steep angles in order to spread the pulse energy over a larger surface area. However, while these techniques have provided some improvements, the improvements are small and insufficient to appreciable extend the lifetime of the optical components. Research by the present inventors and others [30th International Symposium on Microlithography, Session 12, Paper 5754-62, L. Parthier, ArF Immersion Lithozraphy: a new challenge for CaF2 quality (presented, but unpublished in Proc. SPIE, Vol. 5754 (2005)] have confirmed that even uncoated CaF2 surfaces degrade after only a few million pulses when subjected to pulse energies above ˜40 mJ/cm2 using 193 nm excimer radiation. ArF excimer lasers (193 nm) typically operate at average pulse energies 15-20 mJ/cm2. However, local non-uniformities in the beam profile are 2-3 times the average value, thus exceeding the ˜40 mJ/cm2 threshold for damage on-set. The present inventors believe that the damage begins at these local hot spots and then progresses rapidly in the surrounding areas.
Maier et al, have offered solutions as disclosed in U.S. Pat. Nos. 6,466,365, 6,833,949 and 6,827,479 that have significantly extended optical component lifetimes and have allowed laser operation at increased power levels. While the solutions disclosed in the foregoing patents have resulted in a significant extension of optical component lifetime, changes in the optical lithography area require even additional improvements. For example, laser systems operating at 4 KHz, and even 6 KHz, have become available. These systems offer the potential for even higher operating power levels. Furthermore, a new method, immersion lithography, has recently shown great promise [Webb et al. (Corning-Tropel), “Hyper-numerical aperture imaging challenges for 193 nm”, SPIE Proc. Vol. 5754 (2005), pp. 69-79]. Immersion allows higher numerical apertures, resulting in increased resolution, but it also requires that the final element of the objective lens operate immersed in highly purified de-ionized (“DI”) water. Since the immersed element also represents the highest energy density in the system, it must be manufactured from a metal-fluoride crystal in order to avoid the problems encountered with high purity fused silicon dioxide noted above. Metal-fluoride crystals, especially CaF2 crystals, and the metal-fluoride optical coatings which may be present, are slightly soluble in highly purified DI water. This solubility leads to surface degradation, and hence to short useful lifetimes in immersed lithographic applications. Furthermore, even the presence of small amounts of water vapor and/or carbon dioxide as may be present in nitrogen purged environments can greatly accelerate the degradation of metal fluoride crystal optics. Using calcium fluoride as an example, the following equations represent the reactions that can occur.CaF2+H2O+->CaO+2HF(g)CaF2+H2O+CO2->CaCO3+2HF(g)
Similarly, chemical reactions are also possible between water and carbon dioxide and the fluoride materials used for the optical coatings. Regarding immersion optics (for example, a lens), one surface of the lens will be bathed in liquid water which can also contain dissolved CO2 from the atmosphere. For example, LaF3 films are easily oxidized and hydrolyzed forming La—O and La—OH on exposed surfaces; the problem becoming worse as the film porosity increases the surface area [see Taki et al., Thin Solid Films, Vol. 420 (2002), pp. 30-37].
The reaction products shown in foregoing equations absorb sub-200 nm wavelengths, which immediately leads to surface heating under irradiation, and such surface heating accelerates the pace for further surface degradation. As a result, improvements are required for metal fluoride optical elements or components that are used in immersion lithography. The invention disclosed herein prevents or severely limits the chemical reactions illustrated above and thus provides extended lifetimes for uncoated metal-fluoride substrates, and for metal-fluoride coatings, which may be present and operating in nitrogen purged environments, and for metal fluoride optical elements operating in DI water immersed configurations.